An object of conventional signal representation methods is to present a signal in such a manner as to minimize an objective image quality measure based on the mean square error (MSE). These methods include, for example, pulse code modulation (PCM), differential pulse code modulation (DPCM), or a conversion-based method.
Such image quality measures do not correspond to the results obtained when a human observer subjectively compares image qualities. It is known that a human visual system (hereinafter, referred to as HVS) is sensitive to several kinds of image properties when comparing image qualities. More specifically, the HVS cannot distinguish textures having the same higher order statistics when viewing them only preattentively. Thus, some texture synthesis methods are based on synthesizing textures matching higher order statistics of another given texture (See: Patent Reference 1). The texture synthesis methods are used for an image that is to be retouched to fill in a hole or remove an object in a scene. However, in video coding, there is another approach which is to synthesize a plurality of textures rather than to code them directly (See: Non-Patent Reference 2 and Non-Patent Reference 3).
In addition, another conventional method of improving the subjective image quality for the HVS is to sharpen images. A common technique is to use an unsharp mask or local contrast enhancement to render the image shaper.
In the manner as described above, it is possible to improve the subjective image for the HVS by using techniques of synthesizing textures or sharpening images.    Non-Patent Reference 1: J. Portilla and E. P. Simoncelli, A parametric texture model based on joint statistics of complex wavelet coefficients, Int. J. Comput. Vis., vol. 40, no. 1, pp. 49-71, 2000    Non-Patent Reference 2: A. Dumitras and B. G. Haskell, An encoder-decoder texture replacement method with application to content-based movie coding, IEEE Trans. Circuits Syst. Video Technol., 14: 825-840, 2004    Non-Patent Reference 3: P. Ndjiki-Nya, et al., Improved H.264 coding using texture analysis and synthesis, Proc Icip 2003, 3: 849-852, 2003